“See something, say something,” is an advertising campaign from the Department of Homeland Security (DHS) that’s being expanded throughout the country. It asks the public to stay alert, and if people see something suspicious, to report it.

Here in Washington, D.C., the DHS public service announcement sometimes broadcasts over loudspeakers in the Metro subway stations. It blends in with the rumbling arrival and departure of trains, and I don’t take much notice of it.

But at an area restaurant recently, an opportunity arose to follow through on the appeal. What surprised me was how strongly I hesitated to speak up.

It was Labor Day, and it seemed as if half of Washington was dining at the popular Sequoia restaurant on the waterfront in historic Georgetown. The restaurant features a large outdoor terrace that overlooks the Potomac River. My husband and I sometimes take guests there.

And so we did on that unusually temperate, cloudless evening, when we sat at a table for four. At the table to my right, a fashionable young woman dined alone. I couldn’t help but notice her: bleach-blond hair, heels the height of the Washington Monument, and an enormous purse.

About halfway through dinner, one of our guests spoke up. She mentioned that the woman had left about 10 minutes ago, leaving her purse on the table top.

I understood the implication, but dismissed it in my own mind. I’m a chronic forgetter-of-purses. The woman hardly struck me as a terrorist. Still, the bag remained unattended, and in a few more minutes, our guest mentioned it again.

A discussion ensued among us, as well as in my own head. Well, this is Washington. It’s a crowded restaurant. It’s a holiday weekend with many people out and about. And what woman would abandon her purse for that long? If she had forgotten it, wouldn’t she have returned by now? Still, what if there were some reasonable explanation for this; why cause a stir?

Finally, one of our guests took charge and called over the waiter. He said he would check on it, and then disappeared. It was quite a while before he returned with his intelligence report, and during that time, we all became increasingly uncomfortable.

It turns out the woman was chatting over at the bar, out of our line of sight. When we left, she and her over-sized purse had joined what looked to be a large family reunion.

OK, so it was nothing. But what if we had remained silent, and it had been something? What if the New York street vendor who tipped off a policeman about the Times Square bombing attempt had said nothing?

Since 2009, about two dozen Americans have been arrested on terrorism charges, according to Janet Napolitano, the secretary at the Department of Homeland Security. "Homegrown terrorists represent a new and changing facet of the terrorist threat," she said in written testimony this week.